The total ferrous content (in ppm) of a fluid sample can be measured by the response of a coil about the fluid sample. See for example U.S. Pat. Nos. 4,563,644, 4,613,815, 4,926,120, 5,001,424, 5,041,856, 5,315,243, 5,404,100, 5,444,367, 5,811,664, 6,051,970, 7,737,683, 7,784,332, 8,115,478, 8,354,836, 8,522,604, US20060152213, WO2004104561, and WO2007088015 all incorporated herein by reference. The ferrous particles have a magnetic permeability and the permeability of the particles increases the inductance of the coil.
A typical way to measure inductance is to supply a known AC current to the coil and measure the voltage across the coil. Using this method, it is necessary to measure a small change in a large voltage. The measuring circuit should then be extremely stable and have a wide dynamic range. For example, if a resolution of 1 ppm ferrous content is desired, the measuring circuit would have to have a dynamic range of about 14,000,000:1.
Additionally, the coil itself is not very stable with temperature. This is caused by the thermal expansion of the wire and bobbin and the fact that the permeability of the ferrite shield has a large temperature coefficient. Together, these factors probably produce an inductance temperature coefficient of about 500 PPM/° C. Thus a 1° C. change in coil temperature would change the inductance about 70 times as much as the sample. In addition, the temperature coefficient of resistance is about 3900 PPM/° C., which would change the overall impedance about 5500 times as much as the sample.
One solution to this problem is to put two identical coils in series and to use them as a voltage divider. One coil is the active coil and receives the sample while the other identical coil is a dummy coil that never receives a sample. The coil pair is driven with a sinusoidal voltage of about 8V p-p at 22 kHz. The power dissipated by the coil drive can increase the coil temperature by 10-15° C. Both coils increase in temperature by about the same amount, so balance is maintained over temperature.
With the voltage divider, there is still the problem of measuring a small change in a large voltage. This problem is solved by providing each coil with a secondary winding and connecting the secondaries in anti-series. Such a difference coil arrangement allows the nominal output voltage to be reduced to zero. It also allows the output to be increased by using a turns ratio greater than one.
With the introduction of the sample, the voltage at the junction of the primary voltage divider changes from 4 Vp-p to 4.000014 Vp-p for a 100 ppm sample. At the output of the secondaries, the voltage changes from 0 to 68 microvolts.
See also U.S. Pat. No. 9,274,041 incorporated herein by this reference.